V CRUSTACEA— SEKhiORY ORGANS 351 



of the courses of the fibres, and shows itself externally by the formation of lobes. It 

 is probable that the brain of the ancestors of the Crustaceans contained the centres 

 for the unpaired frontal eye, for the anterior antenna and perhaps also for the frontal 

 sensory organs, together with the fibrous connections between these centres them- 

 selves, between these centres and the oesophageal commissures, and lastly the 

 anterior transverse connections of the oesophageal commissures. A higher complica- 

 tion is caused in most of the Crustacea now living (all Malacostraca and many Ento- 

 mostram), first by the occurrence of the paired eyes, and second by the fact that the 

 originally infra-oesophageal ganglia of the posterior antennaj unite with the brain from 

 behind. We can in these cases distinguish three principal regions in the brain. 

 First, an anterior region (fore-brain), with the centres for the unpaired eye (where 

 this persists) and for the paired eyes. The optic nerves of the latter, whose 

 fibres in the brain of the higher Crustacea form a chiasma, enter on each side a 

 frequently very large optic ganglion, which is the largest accessory lobe of the fore- 

 brain. Besides this the fore-brain has, especially in the higher Malacostraca, other 

 lobate formations. Second, a mid-brain which adjoins the fore-brain, and contains 

 the centres for the nerves of the anterior antenna. Third, the hindermost region 

 of the brain (hind-brain), which is formed by the ganglia for the posterior antennje, 

 whose transverse commissure is to be found behind the cesophagus, where it has, 

 as already described, often remained as a separate transverse commissure between 

 the CESophageal commissures. 



Neurochord strands or giant nerve tubes, like those with which we became 

 acquainted in the Annulata, occur in the ventral chord of the Thoracostraca. An 

 intermediate nerve also, of varying extent, has here and there been observed in the 

 ventral chord. 



VI. The Sensory Organs. 

 A. Eyes. 



These are, as a rule, well developed in the Crustacea, and often 

 show a high degree of complexity, especially in the Malacostraca, 

 occasionally also in certain Entomostrccca. Visual organs are either 

 wanting or very much reduced in the adult condition of most of the 

 parasitic and attached Crustacea {Cirripedia), also in many deep-sea 

 forms and in others which live in dark places. Setting on one side 

 a few divergent forms of Crustacean eyes, we can distinguish 2 kinds, 

 which may occur simultaneously in the same animal : the unpaired 

 frontal eye (accessory eye) and the paired lateral eyes (principal eyes). 

 Both belong to the head. The unpaired eye lies above the brain, the 

 paired at its sides. 



The unpaired, eye occurs in the young larval forms of all Crustaceans 

 (Nauplius eye) : it is always retained in the adult Entomostmca, some- 

 times well developed, sometimes in a reduced condition. In the 

 Malacostraca it degenerates in the course of development. From the 

 universal distribution of the unpaired eye in the Entoniostraca and 

 young Malacostraca we may conclude that it was present in the 

 ancestors of the Crustaceans. 



Paired eyes are found in all Malacostraca and many Entomostraca. 

 They are either movable stalked eyes or fixed sessile eyes. The first 

 may be imagined to arise out of unstalked eyes by the raising of that 



